Step 3 Sizing Fleets
©Dr. B. C. Paul 2000
Sizing the Fleet
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We have a confirmed cycle time Why Don’t we calculate the
production and number of Trucks
Because things detract from
production
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•
•
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Operators Don’t run all the time
Trucks Break Down
Loader Breaks Down
We fail to utilize all available time
Trucks bunch up a create random delays
at loader
Calculating Production
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How Many Shifts per Day and
how long?
How Many Days Per Week?
How Weeks Per Year?
Quarries often don’t light haul
roads
• Figure and average of 10 hours/day
• Use 6 days a week
• Nine Months of the year
More Production Calc
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10 hours/day * 6 days/week * 42
weeks/year * 60 minutes per
hour = 151,200 minutes/year
151,200 / 11.28 minute/cycle =
13404 cycles per year
Now factor in labor running
vehicles 50 minutes every hour
• 13,404 * .83 = 11,125 cycles
Production Calculation
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Next Consider that the Truck
will not be Operable all the time
• New trucks with close
maintenance get about 90 to 95%
availability
• Quarries with old trucks are lucky
to get 65%
• 85% was long considered normal
for good maintenance but less
than ideal conditions
Production Continued
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11,125 * 0.85 = 9,456 trips per
year
Now consider that the truck
cannot load itself and will not
be producing when the loader is
down
• In the case of multiple loaders the
calculation is trickier
• Use FEL 85% available
More Production
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9,456 * 0.85 = 8,038 cycles/year
Next Consider Utilized of
Available
• In real life there are times when
equipment and people are
available but cannot be used during a pit blast for example
• We will assume 95% utilized of
available
More on Production
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8,038 * 0.95 = 7,636 cycles
Calculate our draft production
per truck (one factor is still
missing)
• 7,636 * 58 tons = 442,884
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To get 1 million tons this implies
3 trucks will be purchased (We
might also consider 2 adding a third as the
fleet ages)
Matching the Number of
Trucks to the Number of
Loaders
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If there are too few trucks the
loader will sit around idle a lot
of time
If the first truck can get back to
the loader before the other
trucks are serviced you have a
guaranteed que that will add a
fixed delay element to your
truck cycle
Check for Matched
Number
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Formula is
• Truck cycle time / loader service time
• The loader service time is the time the loader
spends filling a truck and if applicable waiting
for it to spot
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11.28 / 3 = 3.76 trucks
If you have fewer trucks system
is called “under trucked”
If you have more “over trucked”
Working Toward
Bunching Evaluation
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We have 3 trucks so we are slightly
under trucked
In real life trucks tend to have a
distribution of actual cycle times causing them to arrive in bunches at
loader
The closer the number of trucks is to
the maximum allowed the more
times you will have idle cycle time
waiting in a que
Computer Aided
Bunching Evaluation
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One way to check bunching is
with full computer simulation
• There are special programming
languages to allow simulation
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Cat did extensive simulations
and then prepared summary
tables of results
We can use the Match Factor
Tables
Calculating Match Factor (No
not the Make-Up Company)
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Formula
• #Trucks * loader time / (# loaders *
truck cycle time
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Plug In
• 3 * 3 /( 1 * 11.28) = 0.8 Match
Factor
• Under Trucked
Cat Match Factor Table
Apply the Match Factor
and Recheck Fleet
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442,884 tons/truck * 0.9 =
398,596 tons/truck per year
With 1 Million tons this does not
change the selection of 3 trucks
If it had we would have needed
to recalculate the number to
trucks and loaders and check
again
Tricks and
Considerations
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There are formulas for truck
production that cover these
factors in various forms
Trick is to count essential
information without double
accounting
• People for example sometimes try
to put bunching into utilized of
available